1. Field of the Invention
The present invention relates to an ion conductor suitable for electrochemical devices, such as fuel cells or sensors.
2. Description of the Related Art
Over the long history of ion conductors, a variety of ion conductors have been developed. In particular, examples of ion conductors for electrochemical devices, such as fuel cells or gas sensors, include zirconia and cerium oxides, which are solid oxide ion conductors. On the other hand, for example SrCe1-xMxO3, CaZr1-xMxO3, and SrZr1-xMxO3 (wherein M is a trivalent element, and 0 less than x less than 1; this is the same in all of the following formulae unless noted otherwise) are known as proton conductors. It also has been reported that oxides of barium and cerium (BaCe1-xMxO3) are mixed ion conductors, in which both oxide ions and protons are conducted simultaneously.
Thus, many ion conductors have been found, but only few have made it into practical use. At present there are only zirconia, used for oxygen sensors, and SrCe1-xMxO3, CaZr1-xMxO3, etc. used for the detection of the hydrogen density in melting furnaces. And in these ion conductors, there are limitations to the environment in which they can be used, and their reliability is insufficient. For example, these ion conductors resolve within 1 to 100 hours in boiling water. This is the same for the mixed ion conductor BaCe1-xMxO3. Among the conventionally known proton conductors of perovskite oxides, there is almost none that is stable in boiling water. Conventionally, stability in boiling water has not been considered for ion conductors of perovskite oxides.
Furthermore, ion conductors of solid oxides often are used at high temperatures, so that a high resistance against thermal shock is required. However, in the conventional ion conductors of perovskite oxides, the mechanical strength is not sufficient, and often cracks develop due to thermal shock.
Thus, it can be seen that ion conductors, and especially proton conductors of perovskite oxides, with high reliability are rare. However, as the development of electrochemical devices such as fuel cells proceeds, there is a strong demand for ion conductors with high reliability, that can be used under harsh environmental conditions.
It is therefore an object of the present invention to provide an ion conductor with high reliability and an electrochemical device using the same.
In order to achieve this object, an ion conductor in accordance with a first aspect of the present invention is a perovskite oxide having the composition BaZr1-xCexO3-p, wherein x is larger than zero but not larger than 0.8, and p is larger than zero but smaller than 1.5.
An ion conductor in accordance with a second aspect of the present invention is a perovskite oxide essentially consisting of Ba, Zr, Ce and O, which substantially conducts protons only.
An ion conductor in accordance with a third aspect of the present invention is a perovskite oxide having the composition BaZr1-x-yCexMyO3-p, wherein M is a trivalent substituent element, x is at least zero but smaller than 1, y is larger than zero but smaller than 1, x+y is larger than zero but smaller than 1, and p is larger than zero (usually at least y/2) but smaller than 1.5, wherein the ion conductor is a substantially single-phase polycrystal of cubic, tetragonal or orthorhombic crystal system whose unit cell axes a, b and c (with axe2x89xa7bxe2x89xa7c) satisfy 0.8386 nm less than a less than 0.8916 nm and b/a xe2x89xa70.90.
This ion conductor is a mixed ion conductor conducting protons and oxide ions (including oxygen ions). This ion conductor has an interatomic distance that is suitable for ion conduction, and a crystal structure that is physically and chemically stable. In this ion conductor, it is preferable that b/axe2x89xa70.90 and c/axe2x89xa70.90, because then its structure is close to that of a physically and chemically stable cubic system.
An ion conductor in accordance with a fourth aspect of the present invention is a perovskite oxide having the composition BaZr1-x-yCexMyO3-p, wherein M, x and y are as in the ion conductor of the third aspect, and wherein the ion conductor is a substantially single-phase sintered product whose density is at least 96% of the theoretical density.
Here, the theoretical density is the density calculated from the lattice constants.
An ion conductor in accordance with a fifth aspect of the present invention is a perovskite oxide having the composition BaZr1-x-yCexMyO3-p, wherein M, x and y are as in the ion conductor of the third aspect, and wherein the ion conductor is a substantially single-phase sintered product with a granular diameter of at least 1 xcexcm and not more than 30 xcexcm.
Here, the granular diameter is the particle diameter of the granules constituting the sintered product.
Resolution in boiling water and resolution due to thermal shock does not only vary with the interatomic bonding strength and crystallinity, but also depends on sintering. In the ion conductors according to the fourth and fifth aspect of the present invention, the density and the granular diameter of the sintered product are controlled in order to increase the reliability.
In the third to fifth ion conductors, it is both possible to include Ce (x greater than 0) or not to include Ce (x=0), but M is essential (y greater than 0).
It is preferable that the element M is at least one element selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y, Sc, Mn, Fe, Co, Ni, Al, Ga and In. It is particularly preferable that M is at least one element selected from the group consisting of Gd, Yb, Y and In.
The present invention also presents an electrochemical device (for example, a fuel cell or a gas sensor) including any of the aforementioned ion conductors as the solid electrolyte.